Multilayer thick film circuits are used as a reliable means of interconnecting high speed, high density integrated circuit packages. These circuits are typically constructed by screen printing and firing alternate conductor and dielectric ink layers on a suitable substrate. The conductor layers are interconnected by depositing a conductor material in the vias of the dielectric layer. The conductor layers are typically fabricated from precious metal conductor inks employing gold, platinum, palladium, or silver, and a low softening point vitreous glass. These conductor inks are very expensive because of the use of the precious metal and, in the case of silver, are susceptible to severe electrolytic migration. The dielectric ink which is used to separate the conductor layers is typically a low softening point vitreous glass frit containing suitable ceramic fillers.
As an alternative to the precious metal conductor inks, copper conductor inks are finding increased use in the electronics industry. Copper is an inexpensive raw material which possesses excellent properties, such as high electrical and thermal conductivities, excellent solderability, and a lower electrolytic migration tendency when compared to silver. The typical copper conductor inks also employ low softening point vitreous glass frits.
Multilayer circuit structures which utilize copper as a conductor material do experience certain problems. The most common failure is caused by the development of electrical shorts due to the interactions between the flux materials of the copper conductor ink and the dielectric ink which take place during the multiple firing steps used to fabricate the multilayer circuit structure. Copper oxide, which forms upon the exposure of the conductor ink to air or an oxidizing media, forms an eutectic mixture with the flux materials of the conductor inks, such as lead oxide and bismuth oxide. This eutectic flux phase migrates through the porous dielectric material, particularly if it contains large modifier ions, such as lead, barium and bismuth. The eutectic flux phase forms a conductive path through the dielectric which shorts the adjacent copper conductive layers.
Copper conductor inks, as well as the dielectric inks, are also susceptible to the entrapment of gaseous materials formed during the repeated firing steps. The organic vehicle used to give the copper conductor or dielectric inks the proper rheology for screen printing outgases during the firing steps. This outgassing organic material can cause both blistering and peeling of the deposited thick films. Outgassing is also responsible for increasing the porosity of the dielectric film which further aggravates the eutectic flux phase migration problem.
Attempts have been made to reduce the above-mentioned problems by formulating dielectric inks which have reduced porosity.
Another approach is to treat both the dielectric and copper conductor inks with an oxidizing or reducing plasma prior to firing. This technique is described in commonly assigned U.S. Pat. No. 4,619,836, entitled "Method Of Fabricating Thick Film Electrical Components", issued Oct. 28, 1986. The plasma treatment removes the carboneous residue of the organic vehicle which is present in conventional ink formulations.
A third approach to both the shorting and blistering problems encountered in conventional copper conductor inks is disclosed by Applicants in their commonly assigned copending U.S. patent application Ser. No. 914,303, entitled "Thick Film Copper Conductor Inks", filed Oct. 2, 1986. Improved copper conductor inks containing devitrifying glass frits are disclosed in this application. These copper conductor inks avoid the use of typical low melting point oxide fluxes, such as lead oxide and bismuth oxide. Since these flux materials are not employed, the eutectic flux phase normally formed with copper oxide is not present and the electrical shorting problem is eliminated. Additionally, since a high softening point devitrifying glass frit is used in the copper conductor inks, the organic vehicles are allowed to outgas before the ink begins to significantly flow and densify.
A need has now developed for inks similar to that disclosed in Applicants' copending application which have increased adhesion.